(1) Field of the Invention
This invention relates generally to rotary type positive displacement pumps, and deals more particularly with a positive displacement rotary pump for achieving high flow rates of incompressible fluids in an underwater environment where a vehicle's acoustical noise must be kept to a minimum.
(2) Description of the Prior Art
In a shipboard environment where torpedoes are to be launched by either a positive displacement ram-type piston, or by a rotary pump system difficulties have been encountered with such prior art systems chiefly for two reasons, first due to the acoustical noise created, and secondly due to the space requirements for such systems generally.
In a ram-type piston system the volume of water to be pumped must be approximately equal to the volume of the weapon which is to be launched. This problem of volume is magnified by the fact that the water piston is powered by an in-line air piston, effectively doubling the length of the overall ejection system required. In addition, as the piston completes its stroke, it must be stopped or decelerated without generating excessive noise. The piston must then be returned to its ready position before a second launch can be achieved.
These factors have led to the development of a rotary impeller pump system to replace the so-called ram or piston-type system. An air turbine has been used to drive the impeller so as to pump seawater behind the weapon to effect its launch. Such a system is considerably smaller than the ram or piston-type system and does permit a gradual deceleration of the pump at the end of the power stroke and does not require any return to a ready position. However, one difficulty with a turbine driven impeller pump launching system can be attributed to the fact that the pump will never reach steady state operation as the weapon will have been launched before the pump reaches this condition. Torpedo tube-weapon exit velocity requirements dictate that the weapon be launched in less than one second.
The development of a quiet system with a pump of the impeller type that will not cavitate while in a start-up or transient mode of operation has encountered problems. Excessive noise has been generated in impeller-type pump systems due to cavitation. Large diameter pumps rotating relatively slowly have been proposed, but the size requirements are such that there is a need for a more efficient and cost effective pumping system to launch weapons from underwater vehicles.